Such presses have been used for drying sludges, wherein aqueous sludge is compressed by means of a screw in a cylinder with a perforated mantle. An adjustable gate may be provided at one end of the cylinder in order to create counter-pressure.
Waste which is to be burnt must be made as dry as possible. The dry-matter content even of waste to be composted should be brought to a level of 40-60%. This is often difficult with known equipment and, for example, when treating normal municipal waste it is even impossible to achieve. To raise the dry-matter content to an adequate level, dryer fibrous material, such as dried peat, must usually be added in relatively big quantities as back-up material to sludges.